JP7556109B2 - Water-based ballpoint pen ink composition - Google Patents

Description

The present invention relates to an aqueous ink composition for ballpoint pens that can eliminate the phenomenon of ink slackening when starting to write immediately after stopping writing (deterioration of initial writing performance).

Conventionally, colorants such as colored resin particles with large particle diameters, such as thermochromic microcapsules, used in water-based ballpoint pens, have low fluidity in ink compared to general dyes and pigments, and therefore have a relatively slow response from a static to a dynamic state or from a dynamic to a static state.
In writing instruments such as ballpoint pens, which require frequent switching of movements and experience large changes in shear force on the ink, these characteristics are easily reflected in the lines drawn, and in particular, there is a phenomenon in which the ink sometimes does not flow easily when starting to write again immediately after stopping writing (deterioration in initial stroke performance).

On the other hand, examples of conventional water-based ink compositions for ballpoint pens that use colorants such as xanthan gum and polyvinyl alcohol include:
(1) A water-based ink for a ballpoint pen, which is composed of at least water, a colorant, and a resin, the resin being xanthan gum and polyvinylpyrrolidone, and the content ratio of the xanthan gum and the polyvinylpyrrolidone being 1:5.7 to 1:7.0 (see, for example, Patent Document 1);
(2) An emulsion polymer made from at least one or more vinyl monomers as a raw material, characterized in that it develops color by containing a color-developing compound and a color developer. This emulsion polymer is used as ink. In Example 2, a gel ink ballpoint pen ink is prepared by adding 50 g of a bright green emulsion polymer fine particle dispersion having an average particle size of 0.17 μm and a viscosity of 3.9 mPa·s (25° C., 150 s ⁻¹ ), 2 g of PVP K-15 (manufactured by ISP Japan, polyvinylpyrrolidone, decolorizing agent, product name), and 0.25 g of xanthan gum and stirring them together (see, for example, Patent Document 2).
(3) A water-based ink composition for a ballpoint pen, comprising at least a shear thinning agent selected from xanthan gum and succinoglycan, polyvinylpyrrolidone, a colorant, a water-soluble organic solvent, and water, in which the content ratio of the shear thinning agent to polyvinylpyrrolidone is 1:1 to 1:5.5 (see, for example, Patent Document 3);
(4) A water-based ink composition for a ballpoint pen, which comprises at least a colorant, water, a water-soluble organic solvent, a shear thinning agent selected from xanthan gum, welan gum, and succinoglycan, a thickening inhibitor, and an ionic substance, the content of the ionic substance in the ink is 1% by mass or more based on the total amount of the ink composition, the content of the water-soluble organic solvent in the ink composition is 10% by mass or less, and the thickening inhibitor is selected from polyvinyl alcohol, polyvinylpyrrolidone, a cellulose-based polymer compound, and a polymer compound having a polyalkylene oxide group, and a ballpoint pen containing the same (for example, see Patent Document 4).
etc. are known.

The above Patent Documents 1 to 4 disclose related technologies to the aqueous ballpoint pen ink composition of the present invention, but Patent Document 1 uses a fluorescent pigment or the like with a small average particle size, and there is no description or suggestion of using colored resin particles with an average particle size of more than 0.5 μm. Furthermore, the problem of the invention is to ensure good ink ejection properties and enable long writing distances by preventing as much as possible wear and deformation of the ball receiving seat due to the rotation of the ball in the ballpoint pen tip, and therefore the problem and configuration of the invention differ from that of the present invention.
The above-mentioned Patent Document 2 uses small thermochromic particles with an average particle diameter of 0.17 μm, and there is no description or suggestion of using colored resin particles with an average particle diameter of more than 0.5 μm. Furthermore, the problem to be solved by the invention is a ballpoint pen ink that can utilize an organic solvent as well as heating as a decolorizing means, and the problem and configuration of the invention differ from those of the present invention.
The above-mentioned Patent Document 3 uses a pigment or the like having a small average particle size, and there is no description or suggestion of using colored resin particles having an average particle size exceeding 0.5 μm. In addition, the problem to be solved by the invention is to suppress the drying and solidification of ink near the pen tip (a test of whether or not good writing can be performed after leaving the pen tip exposed and in a horizontal position at 25° C. for 30 days: dry-up resistance), etc., and the problem and configuration of the invention are different from those of the present invention.
In Examples 6 and 7 of the above-mentioned Patent Document 4, reversible thermochromic pigments (thermochromic microcapsules) having average particle sizes of 2 μm and 3 μm are used, but the problem to be solved by the invention is to test whether good writing is possible after leaving the pen at 50° C. for 30 days with the cap removed (dry-up resistance), etc., and further, the mass ratio of polyvinylpyrrolidone to xanthan gum is outside the range of the present invention, and the problem and configuration of the invention are different from those of the present invention.

As described above, the above Patent Documents 1 to 4 disclose techniques related to the aqueous ballpoint pen ink composition of the present invention, but they differ from the present invention in terms of the problem and configuration of the invention.
The object of the present invention is not to inhibit the drying and solidification of ink near the tip of a pen after it is left uncapped for a long period of time as described in Patent Documents 3 and 4 above (dry-up resistance), but to solve the phenomenon that occurs when colored resin particles with a large average particle size, such as thermochromic microcapsules, are used in aqueous ballpoint pens, and in ballpoint pens which have frequent switching of operations and large changes in shear force, this characteristic is likely to appear in the drawn lines, particularly the phenomenon of ink slackening when starting to write immediately after stopping writing (several minutes later) (reduced initial stroke performance), and the causes of the dry-up resistance and the phenomenon of ink slackening when starting to write of the present invention (reduced initial stroke performance) can be clearly distinguished.

JP-A-10-279875 (claims, examples, etc.) JP 2003-221405 A (claims, examples, etc.) JP 2006-77074 A (claims, examples, etc.) JP 2009-40825 A (Claims, Examples, etc.)

In view of the problems and current state of the prior art described above, the present invention seeks to solve these problems, and aims to provide an aqueous ink composition for ballpoint pens that can solve the problem of ink stagnation when starting to write immediately after stopping writing, i.e., the problem of reduced initial writing performance.

In view of the above-mentioned problems, the present inventors conducted intensive research and discovered that the above-mentioned aqueous ballpoint pen ink composition can be obtained by containing at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum, and polyvinylpyrrolidone, and by setting the mass ratio of the polyvinylpyrrolidone to xanthan gum within a specific range, thereby completing the present invention.

That is, the aqueous ballpoint pen ink composition of the present invention is characterized in that it contains at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum and polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9.
The K value of the polyvinylpyrrolidone is preferably 10 to 40.
The content of the polyvinylpyrrolidone is 0.5 to 5% by mass based on the total amount of the ink composition.
is preferred.
The water-based ballpoint pen of the present invention is characterized by being equipped with the water-based ballpoint pen ink composition having the above-mentioned composition.

The present invention provides an ink composition for an aqueous ballpoint pen that can eliminate the phenomenon of ink slackening when starting to write immediately after stopping writing (deterioration in initial writing performance), and an aqueous ballpoint pen equipped with this ink composition.

Hereinafter, an embodiment of the present invention will be described in detail.
The aqueous ballpoint pen ink composition of the present invention is characterized in that it contains at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum and polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9.

<Colored resin particles>
The colored resin particles used in the present invention are not particularly limited as long as they are composed of colored resin particles, and examples thereof include: 1) colored resin particles in which a colorant consisting of a pigment such as an inorganic pigment such as carbon black or titanium oxide, or an organic pigment such as a phthalocyanine pigment or an azo pigment is dispersed in the resin particles; 2) colored resin particles in which the surfaces of the resin particles are coated with a colorant consisting of the above-mentioned pigment; 3) colored resin particles in which a colorant consisting of a dye such as a direct dye, an acid dye, a basic dye, a food dye, or a fluorescent dye is dyed onto the resin particles; 4) colored resin particles that have been made thermochromic using a leuco dye or the like; and 5) colored resin particles that have been made photochromic using a photochromic dye (compound), a fluorescent dye, or the like that serves as a photochromic dye.
The resin component of the colored resin particles 1) to 3) above may be, for example, at least one selected from polymers or copolymers of acrylic acid, methacrylic acid, acrylic acid esters, methacrylic acid esters, styrene, acrylonitrile, butadiene, etc., benzoguanamine, phenolic resins, epoxy resins, urethane resins, etc., which may be subjected to crosslinking or other treatments as necessary. As a method for coloring these resins, conventionally known techniques such as suspension polymerization and dispersion polymerization are used.

Examples of the thermochromic colored resin particles of 4) above include thermochromic colored resin particles produced by microencapsulating a thermochromic composition containing at least a leuco dye which is an electron-donating dye and functions as a color former, a color developer which is a component capable of causing the leuco dye to develop color, and a color change temperature regulator which can control the color change temperature in the color development of the leuco dye and the color developer, so as to have a predetermined average particle size.
Examples of the microencapsulation method include interfacial polymerization, interfacial polycondensation, in situ polymerization, liquid curing coating, phase separation from aqueous solution, phase separation from organic solvent, melting dispersion cooling, air suspension coating, spray drying, etc., and can be selected appropriately according to the application.For example, in the phase separation method from aqueous solution, the leuco dye, developer, and discoloration temperature regulator are heated and melted, then put into an emulsifier solution, heated and stirred to disperse in oil droplets, and then, as the capsule membrane agent, a resin raw material whose wall membrane is a urethane resin, epoxy resin, amino resin, etc. is used, for example, an amino resin solution, specifically, each liquid such as a methylolmelamine aqueous solution, a urea solution, a benzoguanamine solution, etc. is gradually put in, and then reacted to prepare, and then the dispersion is filtered to produce a thermochromic colored resin particle made of a thermochromic microcapsule pigment.In this thermochromic colored resin particle, the type, amount, etc. of the leuco dye, developer, and discoloration temperature regulator can be suitably combined to set the color development temperature and decolorization temperature of each color to a suitable temperature.
The thermochromic colored resin particles are preferably ones that exhibit reversible thermochromic properties. The ones that exhibit reversible thermochromic properties may be constituted by using various types alone or in combination, such as a thermal decolorization type that is decolorized by heating from a colored state, a color memory retention type that alternately retains a colored state or a decolored state in a specific temperature range, or a thermal color development type that is colored by heating from a decolored state and returns to the decolored state by cooling from the colored state.

Examples of the photochromic colored resin particles of 5) above include photochromic colored resin particles composed of at least one selected from photochromic dyes (compounds), fluorescent dyes, etc., and resins such as terpene phenol resins, and photochromic colored resin particles produced by microencapsulating a photochromic composition containing at least one selected from photochromic dyes (compounds), fluorescent dyes, etc., an organic solvent, and additives such as antioxidants, light stabilizers, and sensitizers, so as to have a predetermined average particle size. As for the microencapsulation method, they can be prepared in the same manner as in the production of the thermochromic resin particles described above.
The photochromic colored resin particles can be colorless in an indoor lighting environment (indoor lighting equipment selected from incandescent lamps, fluorescent lamps, lamps, white LEDs, etc.) and can be colored in an ultraviolet ray irradiation environment (irradiation with light having a wavelength of 200 to 400 nm, or irradiation with sunlight containing ultraviolet rays) by suitably using a photochromic dye (compound), a fluorescent dye, etc.
Each of the colored resin particles 1) to 5) above can be used as a fluorescent pigment, a microencapsulated pigment of a thermochromic pigment, a photochromic pigment, etc. (coloring material). In addition, each of the resin particles 1) to 5) above can be produced by a known production method, and if a commercially available product is available, it can be used.
From the viewpoints of coloring power, dispersion stability, etc., it is desirable to use colored resin particles having an average particle size of 0.5 to 6 μm, and preferably 1 to 5 μm.
In the present invention, the "average particle size" refers to the particle size (D50) at 50% cumulative volume in the particle size distribution calculated based on the volume standard measured by a laser diffraction method. Here, the average particle size can be measured by a laser diffraction method using, for example, a particle size distribution analyzer HRA9320-X100 manufactured by Nikkiso Co., Ltd.

The content of these colored resin particles is preferably 3 to 30% of the total amount of the ink composition, and more preferably 10 to 30%. If the content of these colored resin particles is less than 3%, the desired line density cannot be obtained, and if it exceeds 30%, the writing feel becomes heavy and the lines are prone to smearing, which is undesirable.

<Xanthan gum>
The xanthan gum used in the present invention is used as a thickening agent, and when used in combination with polyvinylpyrrolidone, the effects of the present invention can be achieved.
Usable xanthan gums include commercially available products such as KELSAN S, KELSAN AR, KELSAN T, KELSAN RD, KELSAN ST, KELSAN M, KELSAN HP, and KELSAN ASX manufactured by Sansho Co., Ltd., and Echo Gum and Monart Gum GS manufactured by Dainippon Pharmaceutical Co., Ltd., and at least one of these can be used (either alone or in a mixture of two or more).
The content of these xanthan gums is set depending on the content and content ratio of polyvinylpyrrolidone, which will be described later.

<Polyvinylpyrrolidone>
The polyvinylpyrrolidone used in the present invention is a polymer compound obtained by polymerizing N-vinyl-2-pyrrolidone. In the present invention, the effects of the present invention can be exhibited only by using polyvinylpyrrolidone and xanthan gum in combination and setting the content ratio (mass ratio) of the polyvinylpyrrolidone and xanthan gum within a specific range.
Of the polyvinylpyrrolidone to be used, it is preferable to use one having a K value of 10 to 40.
The K value of polyvinylpyrrolidone is a viscosity characteristic value that correlates with the molecular weight. The K value in the present invention refers to a viscosity characteristic value calculated by applying a relative viscosity value (25°C) measured by a capillary viscometer to the following formula (1):
〔formula〕
K=(1.5logη _rel -1)/(0.15+0.003c)+[300logη _rel +(c+1.5logη _rel ) ² ] ^1/ 2/(0.15c+0.003c ² )...(1)
η _rel : Relative viscosity of polyvinylpyrrolidone aqueous solution to water c : Polyvinylpyrrolidone concentration (%) in polyvinylpyrrolidone aqueous solution

Among the polyvinylpyrrolidones to be used, those having a K value of 10 to 40 are preferably used since they suppress the aggregation of colored resin particles and further improve the dispersion stability.
Specific examples of polyvinylpyrrolidone that can be used include at least one of commercially available products (either alone or in a mixture of two or more) such as PVP K-15 (K value: 13 to 19) and PVP K-30 (K value: 26 to 35) manufactured by ASHLAND Co., Ltd., Pitzcol K-17 (K value: 15 to 19) and Pitzcol K-30 (K value: 27 to 33) manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and polyvinylpyrrolidone K-30 (K value: 27 to 33) manufactured by Nippon Shokubai Co., Ltd.

The content of these polyvinylpyrrolidones is preferably 0.5 to 5% of the total amount of the ink composition, and more preferably 0.5 to 3.0%. If the content of this polyvinylpyrrolidone is less than 0.5%, the effect of the present invention cannot be fully exerted, while if it exceeds 5%, the colored resin particles will tend to aggregate strongly and the dispersion stability will decrease, which is not preferable.

In the present invention, the content ratio of polyvinylpyrrolidone to xanthan gum used, that is, the polyvinylpyrrolidone/xanthan gum (mass ratio) must be 1-9, and preferably 2-8.
If this ratio is less than 1 or exceeds 9, the effects of the present invention cannot be achieved. The reason for setting the blending ratio in this range (1 to 9) is that although polyvinylpyrrolidone and xanthan gum are both ink thickeners, they have different physical properties. When these are used in combination, they do not cancel each other's physical properties, and by suppressing the aggregation of colored resin particles while having shear thinning properties, good fluidity is maintained and initial writing properties are improved, so they are used within the above range.

<Water-based ballpoint pen ink composition>
The aqueous ballpoint pen ink composition of the present invention is characterized in that it contains at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum and polyvinylpyrrolidone, and the mass ratio of the polyvinylpyrrolidone/xanthan gum is 1 to 9. In addition, in addition to the colored resin particles, a general-purpose colorant and a water-soluble solvent are also contained.
Usable colorants include water-soluble dyes and pigments, such as inorganic pigments and organic pigments, which can be used in appropriate amounts as needed, provided that the effects of the present invention are not impaired.
As the water-soluble dye, any of direct dyes, acid dyes, food dyes and basic dyes can be used in an appropriate amount within a range that does not impair the effects of the present invention.

Examples of the water-soluble solvent that can be used include glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, polyethylene glycol, 3-butylene glycol, thiodiethylene glycol, and glycerin;
Ethylene glycol monomethyl ether and diethylene glycol monomethyl ether can be used alone or in combination. The content of this water-soluble solvent is preferably 5 to 40% based on the total amount of the ink composition.

The ink composition for aqueous ballpoint pens of the present invention contains at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum, and polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9. In addition to colorants other than the colored resin particles, water-soluble solvents, and water (tap water, purified water, distilled water, ion-exchanged water, pure water, etc.) as the remainder of the solvent, dispersants, lubricants, pH adjusters, rust inhibitors, preservatives, or antibacterial agents may be appropriately contained within the range that does not impair the effects of the present invention.

Usable dispersants include nonionic and anionic surfactants and water-soluble resins, and preferably water-soluble polymers.
Examples of the lubricant include nonionic lubricants such as fatty acid esters of polyhydric alcohols, higher fatty acid esters of sugars, polyoxyalkylene higher fatty acid esters, and alkyl phosphates, which are also used as surface treatment agents for pigments; anionic lubricants such as alkyl sulfonates and alkyl aryl sulfonates of higher fatty acid amides; derivatives of polyalkylene glycols, fluorine-based surfactants, and polyether-modified silicones.

Examples of pH adjusters include ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, alkali metal salts of carbonates and phosphoric acids such as sodium tripolyphosphate and sodium carbonate, and hydrates of alkali metals such as sodium hydroxide. Examples of rust inhibitors include benzotriazole, tolyltriazole, dicyclohexylammonium nitrite, saponins, and the like. Examples of preservatives or antibacterial agents include phenol, sodium omadine, sodium benzoate, thiazoline compounds, benzimidazole compounds, and the like.
The above-mentioned components such as dispersants, lubricants, pH adjusters, rust inhibitors, preservatives, and antibacterial agents may be used alone or in combination of two or more. In addition, if commercially available products of these are available, they can be used.

The aqueous ink composition for ballpoint pens of the present invention can be prepared by appropriately combining at least the colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum, polyvinylpyrrolidone, a water-soluble solvent, and other components depending on the application of the ink for ballpoint pens, blending them so that the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9, stirring and mixing them with a stirrer such as a homomixer, homogenizer, or disper, and further removing coarse particles in the ink composition by filtration or centrifugation, as necessary.
An aqueous ballpoint pen can be prepared by filling the ink composition for an aqueous ballpoint pen into an aqueous ballpoint pen body equipped with a ball having a diameter of 0.18 to 2.0 mm.
The water-based ballpoint pen body used is not particularly limited as long as it has a ball with a diameter within the above range, and in particular, it is desirable to fill the above water-based ink composition into an ink reservoir tube of a polypropylene tube and finish it into a water-based ballpoint pen refill with a stainless steel tip (the ball is made of a super steel alloy) at the tip.

The water-based ink composition for ballpoint pens of the present invention can be produced in a manner similar to that of other water-based ink compositions.
That is, the aqueous ink composition for a ballpoint pen of the present invention contains at least the above-mentioned colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum, polyvinylpyrrolidone, and other components, and is mixed and stirred so that the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9 using a mixer or the like, and further, for example, a bead mill, homomixer, homogenizer, or the like that can apply strong shear, while setting the stirring conditions to suitable conditions, whereby the aqueous ink composition for a ballpoint pen can be produced.
Furthermore, the pH (25° C.) of the aqueous ballpoint pen ink composition of the present invention is preferably adjusted to 5 to 10 with a pH adjuster or the like from the viewpoints of usability, safety, the stability of the ink itself, and compatibility with the ink container, and is more preferably adjusted to 6 to 9.5.

The aqueous ballpoint pen ink composition of the present invention is loaded into a ballpoint pen equipped with a pen tip portion such as a ballpoint pen tip.
The aqueous ballpoint pen of the present invention may be one in which the aqueous ballpoint pen ink composition having the above-mentioned composition is contained in an ink container (refill) for a ballpoint pen, and a substance that is incompatible with the aqueous ink composition contained in the ink container and has a small specific gravity relative to the aqueous ink composition, such as polybutene, silicone oil, mineral oil, etc., is contained as an ink follower.
The structure of the aqueous ballpoint pen is not particularly limited, and may be, for example, a direct ink type ballpoint pen having a collector structure (ink retention mechanism) in which the barrel itself serves as an ink container and is filled with the aqueous ballpoint pen ink composition having the above-described structure.

The aqueous ballpoint pen ink composition of the present invention thus constituted contains at least colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum, and polyvinylpyrrolidone, and is characterized in that the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9. For example, in the case of large colored resin particles having an average particle size exceeding 0.5 μm, such as thermochromic microcapsules (thermochromic particles), the fluidity in the ink is low compared to general dyes and pigments, and the responsiveness from static to dynamic or from dynamic to static is somewhat slow. In a writing instrument such as a ballpoint pen, which frequently switches between operations and has a large change in shear force on the ink, this characteristic is likely to appear in the drawn lines, and in particular, the phenomenon of ink slackness at the start of writing immediately after stopping writing, i.e., a decrease in initial writing performance, is observed. However, in the present invention, an aqueous ballpoint pen ink composition is obtained that does not decrease in initial writing performance, which is the phenomenon of ink slackness at the start of writing.

Next, the present invention will be explained in more detail with reference to Production Examples 1 to 3 of the colored resin particles used, Examples 1 to 8 of the water-based ballpoint pen ink composition, and Comparative Examples 1 to 4, but the present invention is not limited to the following Examples. In addition, the average particle diameter (D50: μm) of the colored resin particles obtained in Production Examples 1 to 3 was measured using a particle size distribution analyzer HRA9320-X100 manufactured by Nikkiso Co., Ltd.

[Production Example 1: Colorable resin particles; production of thermochromic particles]
1 part by mass (hereinafter simply referred to as "part") of ETAC (manufactured by Yamada Chemical Industry Co., Ltd.) as a leuco dye, 2 parts of bisphenol A as a color developer, and 24 parts of myristyl myristate as a discoloration temperature regulator were heated and melted at 100 ° C to obtain 27 parts of a homogeneous composition. 10 parts of isocyanate and 10 parts of polyol were added as capsule membrane agents to the homogeneous hot solution of 27 parts of the composition obtained above and stirred and mixed. Next, 60 parts of a 12% aqueous polyvinyl alcohol solution was used as a protective colloid to emulsify at 25 ° C to prepare a dispersion. Next, 5 parts of a 5% polyamine was used and treated at 80 ° C for 60 minutes to obtain core-shell type microcapsules.
The average particle size (D50) of the colored resin particles was 3.5 μm.

[Production Example 2: Colorable resin particles; urethane-based black particles]
As an oil phase solution, 12.5 parts by mass of ethyl acetate was heated to 60° C. while 3.5 parts by mass of an oil-soluble black dye (Oil Black 860, manufactured by Orient Chemical Industry Co., Ltd.) and 0.5 parts by mass of a terpene phenol resin (YS Polystar N125, manufactured by Yasuhara Chemical Co., Ltd.) were added and thoroughly dissolved. Next, 8 parts by mass of an isocyanurate-modified product of hexamethylene diisocyanate (TLA-100, manufactured by Asahi Kasei Corporation) was added as a prepolymer to prepare an oil phase solution.
The aqueous phase solution was prepared by heating 200 parts by mass of distilled water to 60° C. and dissolving 15 parts by mass of polyvinyl alcohol (PVA-205, manufactured by Kuraray Co., Ltd.) as a dispersant therein.
The oil phase solution was added to the aqueous phase solution at 60° C., and the mixture was emulsified and mixed by stirring with a homogenizer for 6 hours to complete the polymerization.
The resulting dispersion was centrifuged to obtain colored resin particles (urethane-based black particles). The average particle size (D50) of the colored resin particles was 1.4 μm.

[Production Example 3: Colorable resin particles; urethane-based white particles]
As an oil phase solution, 9.6 parts of myristyl myristate were heated to 60°C, and 2.4 parts of titanium oxide were added and thoroughly dispersed. Next, 9 parts of a methylenediphenyl-4,4'-diisocyanate (3 moles) trimethylolpropane (1 mole) adduct (D-109, manufactured by Mitsui Chemicals) were added, and 2 parts of ethylene glycol monobenzyl ether were further added. As an aqueous phase solution, 15 parts of polyvinyl alcohol (PVA-205, manufactured by Kuraray Co., Ltd.) were dissolved in 600 parts of distilled water, and the oil phase solution was added thereto, and the mixture was emulsified and mixed with a homogenizer to complete the polymerization. The obtained dispersion was centrifuged to recover microcapsules, and a microcapsule pigment was obtained. The average particle size (D50) of the colored resin particles was 0.6 μm.

[Examples 1 to 8 and Comparative Examples 1 to 4]
Using the colorable resin particles of Production Examples 1 to 3, and also using xanthan gum, polyvinylpyrrolidone, and the like in the formulations shown in Table 1 below, each water-based ink composition for a ballpoint pen was prepared by a conventional method.
For the aqueous ballpoint pen ink compositions obtained in Examples 1 to 8 and Comparative Examples 1 to 4, aqueous ballpoint pens were prepared by the following method, and initial writing properties were evaluated by the following evaluation method.
The results are shown in Table 1 below.

(Making a water-based ballpoint pen)
Each of the ink compositions obtained above was used to prepare an aqueous ballpoint pen. Specifically, a refill consisting of a polypropylene ink reservoir tube with an inner diameter of 4.0 mm and a length of 113 mm, a stainless steel tip (carbide alloy ball, ball diameter 0.7 mm), and a joint connecting the reservoir tube and the tip was filled with each of the aqueous inks, and an ink follower mainly composed of mineral oil was loaded into the rear end of the ink to prepare an aqueous ballpoint pen.

[Method of evaluating initial writing ability]
Each of the water-based ballpoint pens obtained above was left uncapped at 25° C. and 50% RH for 60 minutes, after which a straight line was written on PPC paper and the initial writing performance was evaluated according to the following evaluation criteria.
Evaluation criteria:
A: Writing is possible without any problems from the beginning. B: Smearing of more than 0 mm and less than 5 mm is confirmed from the beginning of writing. C: Smearing of 5 mm or more is confirmed from the beginning of writing.

As is clear from the results in Table 1 above, the aqueous ballpoint pen ink compositions of Examples 1 to 8, which are within the scope of the present invention, were confirmed to have excellent initial writing properties compared to Comparative Examples 1 to 4, which are outside the scope of the present invention.
In contrast, looking at Comparative Examples 1 to 4, Comparative Examples 1 and 2 were blends of xanthan gum and polyvinylpyrrolidone alone, and Comparative Examples 3 and 4 were cases in which the mass ratio of polyvinylpyrrolidone/xanthan gum was outside the range of 1 to 9. In these cases, it was confirmed that the initial writing performance was poor from the beginning.
Considering the evaluation results of Examples 1 to 8 and Comparative Examples 1 to 4 above together, it was found that by using the aqueous ballpoint pen ink compositions of Examples 1 to 8, which fall within the scope of the present invention, the initial writing properties were significantly improved for the first time, and specific functional effects could be exerted.

An ink composition suitable for water-based ballpoint pens is obtained.

Claims (2)

The water-based ballpoint pen comprises at least 3 to 30% by mass of thermochromic and/or photochromic colored resin particles having an average particle size of 0.5 to 6 μm, xanthan gum , and 0.5 to 5% by mass of polyvinylpyrrolidone, and the mass ratio of polyvinylpyrrolidone/xanthan gum is 1 to 9 . 2. The water-based ballpoint pen according to claim 1, wherein the polyvinylpyrrolidone has a K value of 10 to 40.